Games the size and scope of Fallout or The Elder Scrolls are never going to be completely free of bugs when they're released for one reason or another. But that said, I'm sure Bethesda's created a new engine (or at least vastly improved the Skyrim engine) for Fallout 4.

(Oddly enough, I was reading about the Creation Engine [the one used for Skyrim and presumably for Fallout 4] and the devs at Bethesda said if they weren't able to develop the Creation Engine, Skyrim wouldn't have come out until the "next generation," meaning Xbox One and PlayStation 4, which means Fallout 4 would've been even more delayed. I'm glad that didn't happen.)